Apparatuses of the aforementioned type, which can be constructed as screw extruders, screw presses, ram presses, etc., are in particular used in plastics processing for the extrusion of plastic materials. However, they are also used for extruding other homogeneous and inhomogeneous plastic materials, e.g. in the production of explosive or powder strands. In the latter case, the pressure and temperature conditions within the extruder, particularly on the extruder head and on the retainer plate must be very accurately controlled, as a function of the components of the material to be processed, so as to ensure that the material does not detonate or deflagrate. Despite carefully respecting the operating parameters, it is not possible to entirely exclude such irregularities. As the material is under feed and shaping pressure and is completely enclosed within the casing, high energy levels are evolved during detonation or deflagration, which can lead to the destruction of machine components or the entire machine and also possibly the area around the machine.
It has therefore been proposed for special applications to fix the retainer plate to the casing by using hydraulic holding clamps, so as to permit the retainer plate to lift in the case of an excessive overpressure in the casing. However, this constructional measure only has the desired effect if the pressure build-up does not occur too rapidly. In addition, the holding clamps only have a limited travel, so that the exit cross-section on raising the retainer plate does not permit a sudden overpressure reduction
However, the problem of an excessive pressure build-up in the casing also exists when extruding other materials. There can be numerous different causes for this, e.g. clogging of the retainer plate in the case of small shaping cross-sections, modification to the material composition and/or the plasticity of the material.
In the case of extruders for pressure-sensitive or temperature-sensitive products, it is known (DE-C-34 31 274), to provide a retainer plate with a linear row of nozzles and to construct the plate in two-part manner with a parting plane located in the nozzle row. One of the two parts of the retainer plate is fixed and the other Connected in articulated manner with the extruded casing and is consequently pivotable. The pivotable plate can be brought by a pneumatic cylinder from the closed into the open position In order to permit raising in a manner dependent on the pressure or temperature, in the extruder is provided a measuring device, which records the pressure or the temperature or some other measured quantity essential for the same and by a control system controls the pneumatic cylinder. This construction is much too slow for rapid pressure rises. It can also not be looked upon as an automatically operating overpressure preventer, because it is a measuring-technical control system. Therefore, it is particularly unsuitable for explosives. In addition, the pneumatic cylinder closes again after opening, without removing any hazard which may be present.